The invention relates to an air damper for a movably supported structural part, in particular in automobiles.
Air dampers of this type have become known, for example, from DE 295 18 171 or DE 43 19 641. The retardation in the piston motion is produced by means of an overflow duct which is defined in an annular groove or annular recess of the piston. A sealing ring is seated in the recess with the axial extension of the sealing ring being smaller than that of the recess. Thus, the sealing ring slides to either of the walls of the recess during the reciprocating motion of the piston. The walls of the recess have grooves so that air may flow from one side of the piston to the other. If the grooves in the bottom and walls of the recess are given appropriate dimensions the throttling effect may be different to achieve different damping efficiencies in the respective directions of displacement.
It is necessary to reach harmonization between the speed at which the piston is displaced and the cross-sectional area or length of the overflow duct. If this is not the case intermediate stoppages may occur during its motion. The structural part oscillates stepwise to the final stop while xe2x80x9cnoddingxe2x80x9d. This problem is even aggravated if the force acting onto the air damper is of a differing magnitude. The best behaviour is obtained if cylinder diameters are large. However, the space required frequently does not exist.
It is also known to provide such air dampers with a tension spring or compression spring which is arranged between a closure of the cylinder and the piston. Both the air damper itself, whether with a spring or without a spring, constitutes an oscillation system which opposes a soft damping action. In addition, the spring forms a potential source of noise.
It is the object of the invention to provide an air damper which produces an efficient ritardation even if accelerations are higher, and does not constitute an oscillation system. Furthermore, the generation of noise is intended to be minimized in the air damper.
This object is attained by the features of the inventive air damper.
The inventive air damper forms a block-like sealing ring made of a thermoplastic elastomer which cooperates with the wall of the cylinder with a predetermined compression force. The sealing ring is coated with a suitable layer which gives the sealing ring a sliding property which produces approximately the same value whether a static friction exists or a sliding friction. In other words, the sealing ring essentially has no breakaway torque. Therefore, so-called stick-slip effects which result in noise generation do not occur. In fact, it is imaginable to achieve uniform friction by the use of lubricants. Lubricants, however, are of different viscosities depending on temperature. In addition, there is a risk that the overflow duct would be reduced in size or would even be occluded by the lubricant. Likewise, lubricants are undesirable if the air brake is employed in the visual range because there is a hazard of contamination.
The inventive air damper helps attain a uniform friction which essentially does not depend on temperature, with the Shore hardness which changes with temperature only having a minimal impact on friction. Thus, there are two factors that influence the damping behaviour and the damping rate of the air damper, which are the pressure difference between the separated cylinder spaces and friction.
Any oscillations that might occur are smoothened by friction, which also needs to be overcome when the air damper requires to be displaced. To make it possible to produce a determined friction the sealing ring has to be a moulded ring which is subjected to a certain compressive force. Such force, however, should be reproducible independently of temperature. In such a case, an undesirable oscillation behaviour does not occur, nor will undesirable noise be generated.
The coating which is used for the sealing ring in this case and which will satisfy the requirements is a polymer coating based on polysiloxanes. The ring is made of a material which has a permanent compression set even at high temperatures.
It frequently happens that the displacement force produced via the air damper is intented to be larger in one direction than it is in the other. This can be done by means of a spring. As an alternative or in addition, according to an aspect of the invention, the bottom of the recess may be conically formed. In this way, a compressive force which is differently high depending on the location of the sealing ring and, hence, differently high friction is produced in the recess.
A coil spring which is intended to impart a preferred direction of displacement to the cylinder may be arranged in the cylinder or outside it. A coil spring, when arranged in the cylinder, may cause some problems. First and foremost, the spring is a source of noise. Thus, a knocking noise might be heard if the air damper is jarred at a quiet travel. The spring starts oscillating while knocking against the piston rod or cylinder wall. A compression spring is relatively instable and, when actuated, rapidly gets at its buckling point. At this point, the spring tends to move away to one side. It also might knock against the piston rod or cylinder wall. In addition, friction noise or whizzing noise may be produced. When the piston rod is actuated friction noise is produced between the shank and turns of the spring. The effect is heavier if surfaces are rough and the spring starts whizzing, which makes itself felt as a squeaking noise.
In order to tackle the problem described, an aspect of the invention provides that the piston rod has formed thereon elongate ribs by which the turns of the spring are engaged if these are relieved. It should be noted here that the spring undergoes an enlargement of the turn diameter essentially in the middle region if the spring is compressed whereas its final turns essentially remain as they were before. However, if the end of the spring also is moved relative to the piston rod care should be taken not to have a clear engagement between the turn of the spring and the piston rod even in the relieved condition. At this point, what requires to be taken into account are the tolerances in manufacturing the piston rod and also the spring. Therefore, it is recommended that although the ribs should extend along the piston rod over a major length the final turns of the spring should not be in engagement with any such rib.
According to the invention, another improvement is that the coil spring is necked down and only the necked portion engages the ribs. It is only the closer-wound middle turns of the spring which engage the whizz removal ribs which, for the rest, may also be made flexible in order to accommodate a major overlap of the turns. For example, this may be accomplished by means of elongate through openings. It is preferred that the spring is of a symmetrical shape so that it does not matter in which position it is slid onto the piston rod.
According to another aspect of the invention, the spring wire is provided with a plastic coating. Preferably, the spring is wound from a plastic coated spring wire. This helps obtain a good sliding effect between the spring and the whizz removal ribs.
Friction noise and rattle noise may also be produced in the guide region between the lid and the piston shank. For clearance reduction, the lid or closure may be provided with suitable spring portions which prevent a rattle noise. In order to reduce friction and the noise that it causes the spring portion ends engaging the surfaces of the piston rod can be ball-shaped.